The present invention concerns a DC/DC converter used in a hybrid system. This hybrid system 1, seen in FIG. 1, includes a fuel cell stack 2, i.e. a plurality of series-connected electrochemical cells. This fuel cell stack 2 is powered by a reducing fuel such as hydrogen and by an oxidising agent such as oxygen. The reaction between the reducing fuel and the oxidising agent generates the fuel cell voltage. The gases derived from the reaction between the reducing fuel and the oxidising agent may be evacuated via recirculation circuits equipped with recirculation pumps. Hybrid system 1 also includes a means of storing energy 6 such as one or several batteries. In the remainder of the description, this means of storing electrical energy will be assumed to be a battery 6 but there is nothing to prevent several batteries being used. This battery 6 provides a battery voltage and is connected in parallel to fuel cell stack 2 so that fuel cell stack 2 and battery 6 are both connected to a variable load 8. This variable load 8 may be, for example, a car engine.
This hybrid system 1 thus includes a DC/DC converter 4 also called a boost or step-up and/or step-down converter comprising two inputs and two outputs. The outputs of fuel cell stack 2 are connected to the two inputs of the DC/DC converter 4, which therefore means that the voltage supplied by the fuel cell stack 2 enters DC/DC converter 4. The connection points of variable load 8 and battery 6 are connected to the two outputs of DC/DC converter 4. DC/DC converter 4 is also arranged to control hybrid system 1 since DC/DC converter 4 is capable of adapting the voltage and current level of fuel cell stack 2 and also that of battery 6. Likewise, the DC/DC converter can regulate the power delivered by fuel cell stack 2.
Indeed, the role of DC/DC converter 4 is to control hybrid system 1 so that battery 6 and fuel cell stack 2 operate together to power load 8. The function of the DC/DC converter is also to distribute the power supplied by the fuel cell stack between the load, which is the electric motor in an automobile application, and the battery, and to maintain the battery charge level at a determined level. The control of hybrid system 1 is of course subject to constraints, which are the voltage and current limits of fuel cell stack 2, the voltage and current limits of battery 6, the state of charge limits of battery 6, the temperature limits that must not be exceeded etc.
It is known that this type of DC/DC converter 4, visible in FIG. 2, includes at least one variable voltage regulator circuit 10 including an input opening on a first filtering capacitor 11 connected in parallel to earth and across a coil 12 series-connected to a diode 14. At the connection point between coil 12 and diode 14, a switching means 13 is connected in parallel and connected to earth. The output of diode 14 is connected to the output where there is arranged a second filtering capacitor 11 connected in parallel to earth. The switching means 13 which may, for example, be a transistor, is frequency controlled by a control signal.
One drawback of this DC/DC converter is that it is bulky since the space taken by the DC/DC converter is proportional to its power. Indeed, the power of this type of DC/DC converter is connected to the impedance of the components. The impedance of a coil or a capacitor is linked to the frequency and to the value of said coil and capacitor. With a defined frequency, the capacitance value of the capacitor and the inductance value of the coil must be increased in order to increase the impedance. This then causes an increase in the size of the components and thus an increase in the size of the DC/DC converter, not to mention an increase in costs.